3-Deoxy-D-arabino-heptulosonic acid 7-phosphate (DAHP) synthase EC 4.1.2.15, the first enzyme of the multibranched pathway that leads to the synthesis of the three aromatic amino acids phenylalanine, tyrosine, and tryptophan, and the the synthesis of several vitamins and the iron chelating compound enterochelin, appears in Escherichia coli in three isoenzymic forms, which are feedback inhibited by one of the three aromatic amino acids each. Cleland-type kinetic analysis of the homogeneous phenylalanine sensitive isoenzyme should soon enable a comparison of kinetic parameters and enzyme mechanisms of isoenzymes from E. coli and other microorganisms. A detailed subunit structural analysis of the E. coli isoenzymes is under way. Studies on the regulation of the synthesis of DAHP synthase from E. coli revealed an Fe3 ion deficiency mediated derepression of the tyrosine sensitive isoenzyme, whereas, the phenylalanine sensitive isoenzyme level remains unchanged in cells grown on minimal salts medium containing various Fe3 ion concentrations in the micron M range. Attempts will be made to elucidate the Fe3 ion deficiency mediated derepression mechanism by mutant analysis BIBLIOGRAPHIC REFERENCES: Berger, F.G. and Herrmann, K.M. "Tryptophan Synthetase alpha (5.7-S), A Novel Molecular Species Formed Within Escherichia coli", J. Bacteriol. 124, 800 (1975). McCray, Jr., J.W. and Herrmann, K.M. "Depression of Certain Aromatic Amino Acid Biosynthetic Enzymes of Escherichia coli K12 by Growth in Fe3 ion Deficient Medium", J. Bacteriol. 125, 608 (1976).